The anonymity. When you don't know who is sending those scary texts, it's really terrifying. Just like in the story where the girl got texts from an unknown number, not knowing who was behind it made it so much scarier.
Threatening words are also a big part. Texts like 'I'll get you' or 'You're in danger' play on our fears of harm. It makes us feel vulnerable, especially when we're alone or at home. For example, if you get a text like that while you're in your bedroom at night, it can send shivers down your spine.
There was a guy who received a text that just had a picture of his own bedroom window from the outside at night. The text said 'Soon.' He was really freaked out. As days passed, he would get more texts with pictures of him in different parts of his house that he hadn't noticed anyone taking. It was a very creepy experience.
One story is about a girl who kept getting text messages from an unknown number. The texts said 'I'm watching you sleep'. Every night, at exactly 3 am, the messages would come. She was so scared that she changed her number, but the new number started getting the same texts.
For me, it's when the text messages seem to predict something bad that might happen to you. For example, if it says 'you'll be in an accident tomorrow' and there's no way to know who is sending it or if it's a real threat. It gives you this sense of impending doom that you can't shake off. You start to question every little thing around you and become paranoid. And if the messages keep coming, it gets even worse, because it feels like there's no escape from this unknown threat.
The sense of helplessness. You can't really stop these texts easily. Blocking the number might not work if they keep coming from new numbers. And you don't know what the sender will do next. For example, in the story of the guy whose car brakes failed after getting the texts, he didn't know how far the sender would go to harm him, and that's really terrifying.
Messages that contain personal details are very frightening. Like when they mention your daily habits, the places you go, or the people you meet. It gives the feeling that someone is closely observing you all the time, which is a very unsettling thought. For example, if a text says 'I know you went to the park with your dog yesterday', it's really scary.
The unknown sender. When you don't know who is sending those terrifying messages, it adds a lot of mystery and fear. It could be anyone or anything.
The anonymity. When you don't know who is sending the text, it's really scary. You can't put a face to the threat or the mystery.
The sense of helplessness. In the hospital story, the nurse couldn't do much about the strange apparitions she saw. She was in a place she was supposed to be in control (her workplace), but still, she was powerless against the supernatural elements. This lack of power to change the situation or escape the horror easily is what makes many elements in true horror stories truly frightening.
The distortion of Christmas symbols is often a really scary element. For example, a possessed Santa Claus or an evil Christmas elf. It takes the things that are usually associated with joy and turns them into sources of fear.